1. Field of the Invention
The present invention relates to the field of radio antennas, and more particularly, to wide frequency coverage vertical, dipole and parasitic array antennas.
2. Description of the Related Art
It is often desired to provide a single antenna having excellent performance over a wide frequency range. In the interest of efficiency and impedance matching, antennas used for radio communication are generally resonant antennas. Unfortunately, resonant antennas by their nature operate over a very narrow range of frequencies. To be resonant at a specific frequency, the antenna must be a certain specific length.
Three commonly used resonant antennas are the dipole, vertical and Yagi-Uda. A dipole antenna is comprised of a single element, usually one half of a wavelength long at the design frequency. It is then usually split at the center where electromagnetic energy is then fed. Vertical antennas are basically dipoles oriented in a vertical plane with one half of the element being driven and the other half removed. The earth is then used as a conductor in its place. Yagi-Uda antennas, frequently referred to as parasitic arrays, are known in the art to provide directional transmission and reception with a high front-to-back ratio as well a low VSWR throughout a very narrow band of contiguous frequencies. Most embodiments of a Yagi-Uda antenna use a single element that is driven from a source of electromagnetic energy. Arrayed with the driven single element are the so-called reflector and director elements that are not driven directly, known as parasitic elements. There is usually only one reflector and one or more directors, with the favored direction of transmitting and reception towards the director elements.
The Yagi-Uda antenna is basically a single frequency device that can be designed to work satisfactorily over a few percent of the center design frequency. However, tradeoffs must be made between gain, front-to-back ratio, and VSWR to allow the antenna to work over this very narrow 3%-4% range. It is often desirable to have a single Yagi-Uda antenna operate in multiple frequency bands. Many radio services have assigned frequencies segregated into bands scattered through the radio spectrum. The amateur radio service is a good example of this, having bands approximately centered at 160M, 80M, 40M, 30M, 20M, 17M, 15M, 12M, 10M, 6M, 2M, etc. Radio amateurs commonly use Yagi-Uda arrays in the 40 m and higher bands. Some prior art antenna designs address multiple bands that cover three of the aforementioned bands, and in some cases five bands, but with very compromised performance. To provide even marginal performance, these antenna designs require large and complex arrays.
To enable wider frequency coverage, three methods have been classically employed. A common method is the use of “traps” that allow one element to function on three bands. Traps are parallel-resonant circuits placed at specific locations on the element to decouple a portion of the element automatically as the antenna operation is changed from band to band. Although multi-element trapped antennas cover multiple frequencies with fewer elements than others designs, they cannot be optimally tuned and there are significant losses associated with traps in all of the elements including the driven element. A trapped Yagi-Uda antenna is a significant compromise in gain, front-to-back ratio, and overall efficiency.
Another method to obtain wider frequency coverage is the use of a so-called log-periodic antenna, in which every element is driven and no element is parasitically driven. This type of antenna can operate over a range of frequencies having a ratio of 2:1 or higher. The antenna impedance varies logarithmically so the VSWR can range as high as 2:1. The log-periodic antenna trades off wide bandwidth for gain and front-to-back ratio. The log-periodic antenna has less gain and less front-to-back ratio than a three element monoband Yagi-Uda antenna yet requires many more elements and a complex feed system.
Yet another method of obtaining wider frequency coverage is the use of an open-sleeve cell type of driven element. This method uses one or more parasitically excited elements placed very close to the driven element. The length of these parasitic elements is usually half that of the driven element. This method results in a wider VSWR bandwidth and the ability to operate on two different frequencies with a single feedline. However, the open-sleeve technique only applies to a driven element. Yagi-Uda antennas require additional dedicated parasitic elements for each anticipated frequency band.